1. Field of the Invention
The present invention relates to exhaust gas cleaning catalyst composition.
2. Description of the Prior Art
As an exhaust gas cleaning catalyst, a three way catalyst which oxidizes CO(carbon monoxide) and HC(hydrocarbon) as well as reduces NOx (nitrogen oxide) has been generally known. The three way catalyst includes Pt (platinum) and Rh (rhodium) carried by Gamma-alumina. The three way catalyst provides a high cleaning efficiency when the air fuel ratio (A/F) is controlled approximately to the theoretical ratio or 14.7.
Meanwhile, in the field of the automobile, it is generally required that the air fuel ratio is increased or the air fuel mixture as an intake gas is controlled as lean as possible to improve the fuel consumption efficiency. In this case, the exhaust gas includes an excessive oxygen, in other words, the exhaust gas is so called a lean composition. As a result, the carbon monoxide and hydrocarbon included in the exhaust gas can be oxidized when passing through the three way catalyst. However, the reduction of Nox is difficult in such a lean mixture so that Nox in the exhaust gas cannot be effectively removed.
Recently, a research is conducted for improving a zeolite catalyst which includes transition metal carried therein by means of ion-exchange. If the zeolite catalyst is used for the exhaust gas cleaning, NOx can be catalytically cracked directly or by virtue of the reducer such as CO, HC and the like which coexist with NOx.
As another type of catalyst, in Japanese Patent Public Disclosure No. 2-265649, laid open to the public in 1990, there is disclosed an A type copper silicate catalyst which includes Al and Cu as metallic component constituting crystal structure.
The zeolite catalyst provides a relatively high catalytic activity around 400 degree centigrade. However, the active thermal range is relatively narrow. The temperature of the exhaust gas of the automobile could be ranged from about 800 to 900 degree centigrade.